Efficient Polytelluride Anchoring for Ultralong-Life Potassium Storage: Combined Physical Barrier and Chemisorption in Nanogrid-in-Nanofiber

Highlights The hierarchical nanogrid-in-nanofiber-structured dual-type carbon-confined CoTe2 nanodots (CoTe2@NC@NSPCNFs) were synthesized via facile templates and an electrospinning approach. Hierarchical nanogrid-in-nanofiber structure effectively suppresses the volume change of CoTe2 and the shuttle of potassium polytelluride (K-pTex) through robust physical restraint and strong chemisorption. CoTe2@NC@NSPCNFs hybrid achieves ultralong lifespan potassium-storage performance over 3500 cycles, and the deep mechanisms underlying the evolution, dissolution, and shuttle of K-pTex have been clearly revealed. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01318-9.

The graphite electrode was prepared using a weight ratio of 8:1:1 for natural graphite/super P/polyvinylidene fluoride (PVDF), using N-Methylpyrrolidone (NMP) as the solvent.After grinding for 30 min, the homogeneous slurry was coated onto aluminum (Al) foils and then vacuum dried at 70 o C for 12 h.The anion-storage performance of the graphite cathodes was characterized by assembling K-graphite potassium-based dual-ion batteries (PDIBs) with 5 M KFSI in EC/DMC (1:1, v/v) as the electrolyte, and the applied voltage window ranged from 3.2 to 5.25 V.The mass loading of active material in the graphite cathodes was around 1.5−2.0mg cm −2 .It is worth noting that the CoTe2@NC@NSPCNFs anodes were pre-potassiated before the PDIBs were assembled.The CoTe2@NC@NSPCNFs anode was first cycled at 0.1 A g −1 for 5 cycles in half cells (counter electrodes: K metal), after which the half cells were discharged to 0.01 V.The CoTe2@NC@NSPCNFs//graphite PDIBs were assembled by using the prepotassiated CoTe2@NC@NSPCNFs as the anode, graphite as the cathode, and 5 M KFSI in EC/DMC as the electrolyte within the voltage window of 1.0−5.25 V.The weight ratio of 3:1 (the active materials in the cathode and anode) was designed to keep the cathode-to-anode capacity ratio around 1.0-1.1.
The preparation of the KPB cathode is similar to that of the graphite cathode, but the active material (KPB), Super P, and PVDF binder are prepared in a mass ratio of 6:3:1.The performance of KPB cathode in half cells was evaluated by using 3 M KFSI in DME as the electrolyte and K metal as counter electrode under voltage window ranging from 2.0 to 4.0 V. To improve the cycling stability of CoTe2@NC@NSPCNFs//KPB full cells, CoTe2@NC@NSPCNFs electrodes were firstly pre-potassiated in half cells for 5 cycles at 0.1 A g -1 and further discharged to 0.01 V.
The CoTe2@NC@NSPCNFs//KPB full cells were obtained with 2032 coin and pouch cells by utilizing CoTe2@NC@NSPCNFs anode, KPB cathode, the electrolyte of 3 M KFSI in DME, and Whatman glass fibers serving as the separator within the voltage window of 0.1−3.6V.The CoTe2@NC@NSPCNFs anode was directly coupled with the KPB cathode at the mass ratio of 5:1 according to the capacity matching.All the above working and counter electrodes were cut into circle pieces with a diameter of 12 mm.Cyclic voltammetry (CV) measurements were performed on an Autolab instrument (PGSTAT 302) at a scan rate of 0.1 mV s −1 .Electrochemical impedance spectroscopy (EIS) was conducted on an electrochemical workstation (Autolab 302N) from 1×10 5 to 0.1 Hz.The galvanostatic charge/discharge tests were conducted on a Neware battery test system (CT-ZWJ-4'S-T-1U, Shenzhen, China).Galvanostatic intermittent titration technique (GITT) tests were conducted by discharging and charging the cells at 0.02 A g −1 for 30 min with a rest interval of 2.5 h in the range of 0.01 to 3.0 V.The volumetric specific capacity was calculated based on the tap density of CoTe2@NC@NSPCNFs (2.72 g cm −3 ).For the electrolytic cell of in-situ ultraviolet-visible (UV-vis) measurements, the working electrodes were prepared by mixing 80 wt% of the active materials (pure CoTe2, CoTe2@NSPCNFs, or CoTe2@NC@NSPCNFs), 10 wt% of PVDF, and 10 wt% of super P with NMP as the solvent.The slurry was uniformly coated onto Al foil and dried at 70 o C for 24 h under a vacuum.K metal and 3 M KFSI in DME were used as the counter/reference electrode and the electrolyte, respectively.

S3 DFT Simulations
The Vienna Ab initio Simulation Package (VASP) was employed to perform all density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE)functional.The projected augmented wave (PAW) potentials were adopted to describe the ionic cores and take valence electrons into account using a plane-wave basis set with a kinetic energy cut-off of 400 eV.The DFT-D3 empirical correction method was employed to describe van der Waals interactions.All crystal structure relaxations were conducted until the residual force acting on each atom was less than 0.05eV/Å.Additionally, the requirement for self-consistent calculation convergence was set at 10 −5 eV.A Monkhorst-Pack k-grid of 1 × 1 × 1 was applied for all the calculations.The adsorption energy (Ea) was calculated by the equation: Ea = E(slab + KxTey) -E(slab)-E(KxTey), where E(slab + KxTey) and E(slab) are the total energy of the surface slab with and without KxTey, respectively, and E(KxTey) is the total energy of the KxTey molecule.

S4 Calculation Process for the CoTe2 Content in CoTe2@NC@NSPCNFs from TGA Analysis
The content of CoTe2 in CoTe2@NC@NSPCNFs was characterized by thermogravimetric analysis (TGA) and analyzed based on the weight loss from carbon combustion (Figure S4A) and the weight increment from the oxidation of elemental CoTe2 to Co2Te3O8 (2CoTe2 + 5O2 + C → Co2Te3O8 + CO2↑ + Te↑) (Fig. S4b) [S1, S2].Therefore, the content of CoTe2 in CoTe2@NPCNFs@NC could be calculated by the following equation: where  CoTe 2 and   2  3  8 are the molecular weights of CoTe2 and Co2Te3O8, respectively.The CoTe2 content in the composite was calculated to be ~65.9wt%.

S5 Calculation Process to Determine the Capacitance Effect and Pseudocapacitive Contribution
The capacitance effect can be determined from the curve, according to the relationship between measured peak currents (i), and scanning rates (v), as follows: where a and b are the fitting parameters, and i and v represent the peak current and scan rate, respectively.The capacitive behavior could be estimated using the b value, which is the slope of the "log i vs. log v" plot [S2].For diffusion-controlled behavior, the b-value approaches 0.5, while for a surface capacitance-dominated process, it is close to 1.0 [S3, S4].Furthermore, the pseudocapacitive contribution can also be calculated by the following equation: where k1v and k2v 1/2 represent the pseudocapacitive contribution and the ionic diffusion contribution, respectively.

S6 Details of the Diffusion Coefficient (DK + )
The galvanostatic intermittent titration technique (GITT) tests were performed by discharging or charging the cells for 30 min at 20 mA g −1 , followed by a 2.5 h relaxation in the range of 0.01 to 3.0 V.The diffusion coefficient can be worked out by solving Fick's second law according to Eq. (S5).
where τ is the duration of the current impulse (s), and mB, VM, MB, and S are the mass, the molar volume of the active material, the molar mass, and the area of the electrode, respectively.ΔES represents the quasi-thermodynamic equilibrium potential difference between the potentials before and after the current pulse.ΔEτ is the potential difference during the current pulse [S5].Fig. S9 Cycling performance of NC@NSPCNFs in the CoTe2@NC@NSPCNFs electrode at a current density of (a) 1.0 and (b) 2.0 A g −1 Fig. S10 EIS curves of (a) the CoTe2@NC@NSPCNFs, (b) CoTe2@NSPCNFs, and (c) pure CoTe2 electrodes before and after different cycles at 0.1 A g −1 .The insets in (a, b, c) are their corresponding equivalent circuits.SEM images of (d, e) the CoTe2@NC@NSPCNFs, (f, g)

S7 Supplementary Figures and Tables
CoTe2@NSPCNFs, and (h, i) pure CoTe2 electrodes before and after 50 cycles at 0.1 A g −1 , respectively.Insets are their corresponding TEM images It can be clearly seen from Fig S10a that the charge transfer resistance (Rct) of the CoTe2@NC@NSPCNFs electrode decreases and stabilizes after 50 cycles, which is attributed to the robust nanogrid-in-nanofiber skeleton that can withstand the strain-induced by the volume change, well maintain the nanostructure, and facilitate the formation of a stable SEI layer during cycling (as exhibited in Fig S10d, e).However, the Rct of the CoTe2@NSPCNFs and pure CoTe2 electrode decreases after 5 cycles and then significantly increases after 50 cycles (especially for the pure CoTe2 electrode), which is ascribed to the huge volume change that occurs during the potassiation/depotassiation processes and eventually leads to severe pulverization of the structure, even the collapse of the conducting network after cycling (as displayed in Fig S10f-i).
Fig. S11 (a) The relationship between log i and log v of the CoTe2@NC@NSPCNFs electrode, where i is the peak current, ν is the scan rate, and b is the slope of log (i) vs. log (ν), and (b) the capacitive contribution (red) of the CoTe2@NC@NSPCNFs electrode at a scan rate of 2.0 mV s −1 Fig. S12 Galvanostatic intermittent titration technique (GITT) curves of the CoTe2@NC@NSPCNFs, CoTe2@NSPCNFs, and pure CoTe2 electrodes Fig. S13 In-situ XRD patterns of the CoTe2@NC@NSPCNFs electrode during the first cycle Fig.S14 (a, b) TEM and (c-e) elemental mapping images of the CoTe2@NC@NSPCNFs electrode discharged to 0.01 V Fig. S15 (a, b) TEM and (c-e) elemental mapping images of the CoTe2@NC@NSPCNFs electrode charged to 3.0 V Fig. S16 XPS spectra of Co 2p and K 2p were analyzed after the initial discharged and charged states of the CoTe2@NC@NSPCNFs electrode: (a, e) discharge to 0.8 V, (b, f) discharge to 0.4 V, (c, g) discharge to 0.01 V and (d, h) charge to 1.5 V Fig. S17 In-situ EIS curves and the corresponding impedances of the CoTe2@NSPCNFs@NC electrode during the initial cycling Fig. S18 In-situ Raman spectra and the corresponding contour plot of the CoTe2@NSPCNFs@NC electrode during the initial cycling Nano-Micro Letters S11/S18 As shown in Fig. S18, the potassium-storage behavior of the CoTe2@NC@NSPCNFs electrode was further analyzed by in-situ Raman spectroscopy.During the discharge process, the D and G bands gradually redshift, which corresponds to the charge transfer effects after the K + intercalation.Notably, the intensity of the D band gradually becomes weak and the value of the ID/IG decreases from 1.66 to 1.21, which can be attributed to the introduction of K + onto the defective sites, thus reducing the optical skin depth.Impressively, the D and G bands, including the ID/IG value, can return to the original state during the charging process, suggesting the excellent structural stability of the CoTe2@NC@NSPCNFs electrode.It can be seen from Fig. S24a that the graphite cathode exhibits a capacity of 87.4 mAh g −1 after 100 cycles at 0.1 A g −1 .Moreover, the rate capability of the graphite cathode was studied, and the results are shown in Fig. S24b.The graphite cathode exhibits high reversible capacities of 99.3, 76.2, 65.5, 57.4, and 45.0 mAh g −1 from 0.1 to 0.5 A g −1 , respectively.Fig. S25 (a) The charge-discharge profiles and (b) cycling performance of the CoTe2@NC@NSPCNFs anode at 0.1 A g −1 with the electrolyte consisting of 5M KFSI in EC/DMC As exhibited in Fig. S25, the initial discharge/charge capacities of the CoTe2@NC@NSPCNFs electrode are 566.7/327.3mAh g −1 with a Coulombic efficiency (CE) of 57.2%.In addition, the CoTe2@NC@NSPCNFs electrode delivers a capacity of 283.8 mAh g −1 after 200 cycles at 0.1 A g −1 .
Fig. S26 Selected galvanostatic charge/discharge profiles of the CoTe2@NC@NSPCNFs//graphite PDIBs at different current densities As shown in Fig. S28a, the KPB cathode delivers a stable specific capacity of 56.5 mAh g -1 after 250 cycles at 0.5 A g -1 .Furthermore, the charge and discharge voltage plateaus are located at 3.45 V and 3.28 V, respectively, indicating that the KPB electrode is an appropriate cathode for K-ion full cells (Fig. S28b).

Fig. S29
Electrochemical performance of the CoTe2@NC@NSPCNFs//KPB full cell: (a) Cycling performance at 0.1 A g −1 , (b) the corresponding charge/discharge profiles at 0.1 A g −1 , and (c) cycling performance at 0.5 A g −1 (Inset: Photograph of LED arrays was powered by one CoTe2@NC@NSPCNFs//KPB full cell) It can be found from Fig. S29a that the CoTe2@NC@NSPCNFs//KPB full cell delivers a capacity of 93.8 mA h g -1 after 100 cycles at the current density of 0.1 A g -1 .Moreover, the average discharge plateau of CoTe2@NC@NSPCNFs//KPB full cells is estimated to be 2.2 V under the voltage window of 0.1~3.6V according to the voltage profiles in different cycles (Fig. S29b).Furthermore, the CoTe2@NC@NSPCNFs//KPB full cell shows a specific capacity of 81.9 mAh g −1 after 200 cycles at 0.5 A g −1 (Fig. S29c).Impressively, the CoTe2@NC@NSPCNFs//KPB full cells can powder the light emitting diode (LED) array with the label of "K + ", which indicates the great potential of CoTe2@NC@NSPCNFs anode for practical applications.Table S3 Atomic contents of pyridinic-N (N-6), pyrrolic-N (N-5), and graphitic-N (N-Q) in CoTe2@NC@NSPCNFs Nitrogen type N-6 N-5 N-Q Atomic content (%) 43.5 43.3 13.2

PDIBs
The range of operating voltage (V)

Fig. S19
Fig.S19The electrolytic cell for the collection of in-situ UV-vis absorption spectra.

Fig. S24
Fig. S24 The electrochemical performance of the graphite cathode with 5M KFSI/EC/DMC electrolyte: (a) the cycling performance at 0.1 A g −1 , and (b) the rate capability from 0.1−0.5A g −1

Fig
Fig. S27 (a) The XRD pattern and (b) SEM image of KPB

Table S1
The density of typical transition metal tellurides

Table S2
The element content of CoTe2@NC@NSPCNFs based on XPS analysis

Table S5
Potassium-storage performance of the CoTe2@NC@NSPCNFs electrode compared with other TMTes materials in previous reports

Table S7
The fitted resistances from the EIS curves of CoTe2@NSPCNFs electrode before and after different cycles

Table S8
The fitted resistances from the EIS curves of pure CoTe2 electrode before and after different cycles